Evaluation of long-term supplementation of a direct-fed microbial and enzymatically hydrolyzed yeast cell culture product on feedlot growth performance, efficiency of dietary net energy utilization, heat stress measures, and carcass characteristics in beef steers

Abstract The objective of this research was to determine the influence of long-term supplementation (258 d) of a direct-fed microbial (DFM) and yeast cell wall (YCW) product used alone or in combination on growth performance, dietary net energy utilization, and carcass characteristics in beef steers finished under climatic conditions in the Northern Plains (NP). Single-sourced Charolais × Red Angus steers [n = 256; body weight = 246 ± 1.68 kg] were blocked by pen location in a 2 × 2 factorial arrangement of DFM and YCW. Steers were administered a series of diets common to the NP and administered ractopamine hydrochloride (RH; 300 mg/kg) during the last 28 d of the finishing phase. Steers were vaccinated and poured at processing and individually weighed on days 1, 14, 42, 77, 105, 133, 161, 182, 230, and 258. Temperature–humidity index (THI) was calculated during RH supplementation. For 98% of the experiment, the THI was lower than 72 and thus cattle were not under high-ambient temperature. On days 1, 2, 21, and 22 of RH supplementation, respiration rates (RR), and panting scores (PS) were determined before and after AM and PM feedings (0700 h, 1100 h, 1400 h, and 1700 h). A DFM + YCW interaction was noted for the proportion of steers categorized as PS 2.0 at 1100 h on day 21 (P = 0.03) and RR on day 21 at 1400 h (P = 0.02). Control steers had a greater proportion of PS 2.0 compared to DFM or YCW steers (P ≤ 0.05), while DFM + YCW steers did not differ from others (P ≥ 0.05); DFM + YCW steers had greater (P < 0.05) RR compared to DFM steers, while control and YCW steers did not differ from others (P ≥ 0.05). No DFM + YCW interactions or main effects (P ≥ 0.05) were observed for cumulative growth performance measures. However, YCW steers had 2% lower (P = 0.04) dry matter intakes compared to steers not fed YCW. No DFM + YCW interactions or main effects (P ≥ 0.05) were observed for carcass traits or liver abscess severity. However, a DFM + YCW interaction (P < 0.05) was noted for the distribution of USDA yield grade (YG) 1 and Prime carcasses. Control steers had a greater proportion (P < 0.05) of YG 1 carcasses compared to other treatments. DFM+YCW steers had a greater proportion (P < 0.05) of USDA Prime carcasses compared to DFM or YCW but were similar to control steers, which were also similar to DFM or YCW. Overall, the use of DFM and YCW alone or in combination had minimal effects on growth performance, carcass traits, and heat stress measures in steers finished in NP climatic conditions.


INTRODUCTION
Antimicrobials (i.e., chlortetracycline) have been used in livestock feeds in an attempt to control bovine respiratory disease (BRD) symptoms and pathogenic bacteria that reside in the gastrointestinal tract of cattle that appear healthy (i.e., asymptomatic). However, certain antimicrobials have been suggested to cause antimicrobial-resistant pathogenic populations that could pose risks to public health (Ohta et al., 2019). Therefore, on January 1, 2017, all medically important antimicrobials to human medicine were listed in the Veterinary Feed Directive (FDA, 2022). Since that time, there has been increased research in the investigation of alternative feed additives to feed cattle during all phases in the feedlot. Microbial feed additives have been used to combat health and nutritional challenges. These feed additives include Bacillus subtilis-based direct-fed microbials (DFM) and have been shown to reduce harmful pathogenic bacteria in the gastrointestinal tract (Krehbiel et al., 2003;Broadway et al., 2020) and improve dry matter intake (DMI) and average daily intake (ADG) during the receiving period (Smock et al., 2020). Prebiotic and probiotic feed additives with enzymatically hydrolyzed yeast cell wall (YCW) components of Saccharomyces cerevisiae have also improved health status and immune responses due to enhanced ruminal pH and ruminal fiber digestion (Salinas-Chavira et al., 2015). In another study, YCW supplementation helped to reduce BRD during a 30-d preconditioning period, as well as increased ADG and feed efficiency (Silva et al., 2018). These benefits may be exacerbated by the combination of probiotics and prebiotics (Colombo et al., 2021).
It is well known that the receiving period in the feedlot is one of the most stressful events in a calf's life and can cause concerns about animal welfare and productivity (Duff and Galyean, 2007;Arthington et al., 2008). The positive effects on the growth performance of cattle that receive probiotics and/or prebiotics are commonly observed in stressed cattle, such as newly received cattle, cattle under poor management practices, or cattle under high-ambient heat load (Duff and Galyean, 2007;Silva et al., 2018;Colombo et al., 2021). In general, those benefits have been observed during short periods of supplementation (i.e., <90 d). Therefore, subsequent events that occur throughout the remainder of the time at the feedlot can also raise concerns.
A main characteristic of cattle that are finished in the Northern Plains (NP) is that for the majority of the time, there are favorable climatic conditions (without high heat ambient load) for finishing. Since the permanence of the cattle in the feedlot generally exceeds 200 d, extreme environmental stress (i.e., cold vs. heat stress) may occur and could hinder overall feedlot performance. Research has been conducted on management strategies to help mitigate environmental stress at both extremes (Mader, 2003;Smerchek and Smith, 2020). Recent research has shown that YCW may also help to reduce the impact of environmental stress and improve DMI and ADG during increased ambient temperatures (Salinas-Chavira et al., 2015;Sanchez-Mendoza et al., 2015). However, in non-stressed animals (or adapted cattle), the magnitude of the response of probiotics and prebiotics could be minimal (Uyeno et al., 2015).
The information about the effects of prebiotics, probiotics, and their combination during long-term supplementation (adapted cattle to feedlot conditions) is scarce. Thus, this information would be valuable to determine the strategies of supplementation. For this reason, the objective of this experiment was to determine the influence that a DFM and YCW fed alone or in combination have on animal growth performance, efficiency of dietary net energy (NE) utilization, carcass characteristics, and liver abscess (severity and prevalence) in beef steers fed in confinement from weaning to harvest under NP climatic conditions.

Institutional Animal Care and Use Approval
This study was conducted at the Ruminant Nutrition Center (RNC) in Brookings, SD, USA. The animal care and handling procedures used in this study were approved by the South Dakota State University Animal Care and Use Committee (Approval Number: 2009-044A).

Climatic Variables, THI, Respiration Rate, and Panting Score Measures
Climatic variables (ambient temperature, relative humidity, and wind speed) were obtained every 30 min from a weather station (VANTAGE PRO2 PLUS, Davis Instruments, Hayward, CA) located at the RNC throughout the experimental period (October 2020 through July 2021). The temperature-humidity index (THI) was calculated using the following formula: THI = 0.81 × ambient temperature + [relative humidity × (ambient temperature − 14.40)] + 46.40 (Hahn, 1999). These measurements were taken in order to verify that animals were not under stress due to a high environmental heat load.
At the initiation (days 1 and 2) of RH supplementation and the during last high ambient temperature days near the end of RH supplementation (days 21 and 22), steers were evaluated for respiration rates (RR) and panting scores (PS) at 0700 h (before AM feeding), 1100 h (after AM feeding), 1400 h (before PM feeding), and at 1700 h (after PM feeding). The respiration rate was determined on three steers per pen by a single evaluator. RR, expressed as breaths per minute (BPM) was calculated according to the following equation: BPM = 600/s required for 10 inward and outward flank movements (Hales et al., 2014). PS were based upon the following scoring system (Hales et al., 2014): 0 = No panting; 1 = Slight panting, mouth closed, no drool, easy to see chest movement; 2 = Fast panting, drool present, no open mouth; 2.5 = As for 2, but occasional open mouth panting, tongue not extended; 3 = Open mouth and excessive drooling, neck extended, head held up; 3.5 = As for 3 but with the tongue out slightly and occasionally fully extended for short periods; 4 = Open mouth with tongue fully extended for prolonged periods with excessive drooling. Neck extended and head up; 4.5 = As for 4 but head held down. Cattle "breathe" from the flank. Drooling may cease.
A single person made the visual evaluation of the 32 pens at each evaluation time point. Notes of the number of animals in each respective panting category within the pen were taken. Since the pen was considered the experimental unit, no individual animal identification was recorded. PS are expressed as a percentage of the cattle in each pen expressing that specific panting score. For example, if 6 out of 8 steers were determined to have a PS of 0 and 2 out of 8 steers had a PS of 1, then 75% of the cattle in each pen would have a PS of 0, and 25% would have a PS of 1.

Cattle Management and Dietary Treatments
Single-sourced, newly weaned Charolais × Red Angus steers, (n = 302) from Western South Dakota, were transported approximately 513 km to the RNC in October 2020. Upon arrival, steers were group housed (10 steers/pen) in 7.62 m × 7.62 m concrete surface pens and were offered long-stem grass hay and ad libitum access to water.
The morning following arrival, all steers were subjected to an individual body weight (BW) measurement used for allotment purposes, application of a unique identification ear tag, vaccination against viral respiratory diseases (Bovishield Gold 5, Zoetis) and clostridial species (Ultrabac 7/Somubac, Zoetis), and administration of pour-on moxidectin (Cydectin, Bayer) according to label directions. Due to a known health history of steers from this ranch, no metaphylaxis was administered upon arrival. The afternoon following initial processing, a subset of steers (n = 256; BW = 246 ± 21.8 kg; selected based upon temperament, health, and uniformity of BW) was allotted to treatment pens (n = 8 pens/treatment with 8 steers/pen; 32 pens total). Steers were randomly assigned to treatments using a 2 × 2 factorial arrangement of factors DFM and YCW. The resulting treatments included: 1) Fed no DFM and no YCW (Control); 2) Fed the DFM and no YCW (DFM; Certillus CP B1801 Dry, Bacillus subtilis and Lactobacillus plantarum based DFM; Arm & Hammer Animal Nutrition; 28 g/steer·d −1 ); 3) Fed no DFM and yes YCW (YCW; Celmanax, enzymatically hydrolyzed yeast component of Saccharomyces cerevisiae; Arm & Hammer Animal Nutrition; 18 g/steer·d −1 ); and 4) Fed the DFM and the YCW (DFM+YCW; Certillus 28 g/ steer·d −1 and Celmanax 18 g/steer·d −1 of Celmanax).
The morning following processing, the first four replicate pens (pens 1-16) were weighed and treatment diets were initiated. The following morning, the last four replicate pens (pens 17-32) for each treatment were weighed and treatment diets were initiated. Two weigh days were used in order to ensure cattle in the last four replicate pens would not be confounded by time off feed due to the processing time. Each subsequent weigh day and processing event was handled in a similar manner in that the first four pen replicates for each treatment were worked the day prior to the last four pen replicates.
The initial on-test BW was the average of the two BW measures collected at processing and the day-test diets were initiated and reduced by 4% to account for digestive tract fill. Throughout the remainder of the study, individual BWs were recorded on days 14, 42, 77, 105, 133, 161, 182, 230, and 258. In order to monitor growth performance, steers were weighed at 28 ± 7 d intervals. All steers were fed in a small pipe and cable pens with concrete bunks (95.25 linear cm of bunk space/steer) and pen floors (7.25 m 2 /steer); all pens are equipped with automatic heated waters.

Feeding Management
All diets contained monensin sodium (Rumensin-90, Elanco Animal Health) at 27.6 mg/kg during the receiving and growing phase and 33.1 mg/kg during the finishing phase; all diets were fortified with vitamins and minerals to meet or exceed nutrient requirements for growing and finishing beef steers (NASEM, 2016).
Supplements for dietary treatment inclusion were manufactured every 30 d at the SDSU feed mill in Brookings, SD. Ingredients were analyzed weekly for dry matter (DM) content and composited monthly for nutrient determination. Actual diet formulation was based upon weekly DM determination and feed batching records along with energy content (Preston, 2016) which is presented in Table 1. All steers were subjected to a 42-d receiving period. On study day 42, steers were weighed, and implanted with a Synovex-S (Zoetis) implant containing 200 mg progesterone and 20 mg estradiol benzoate (EB). Steers were fed the growing diet until day 112 where they were transitioned using two intermediate diet steps to the finishing diet that was fed until harvest (Table 1). The terminal implant (200 mg trenbolone acetate and 28 mg EB; Synovex Plus, Zoetis) was administered 97 d prior to harvest (day 161). Implant retention checks (days 77 and 182) were conducted by a trained technician. One steer from DFM + YCW was identified as having a missing implant; at this time the appropriate implant was re-administered, and no other severe implant site anomalies were noted on day 77 or day 182. Ractopamine HCl (RH) was fed at a rate of 300 mg/steer·d −1 for the final 28 d on feed (day 230 of Soybean hull carrier supplement with either 0 g/steer (CON), Certillus at 28 g/steer·d -1 (DFM), Celmanax at 18 g/steer·d −1 (YCW), or Certillus at 28 g/ steer·d −1 and Celmanax at 18 g/steer·d −1 (DFM + YCW). study). Throughout the entire study, steers were fed twice daily (0800 h and 1400 h) in a 50:50 split. Bunks were managed according to a slick bunk management system to allow for ad libitum access to feed. Feed was manufactured in a stationary mixer (2.35 m 3 ; four pens fed per batch), all ingredients were added into the mixer to the nearest 0.45 kg, and feed was delivered to each pen separately (weighed out of the stationary mixer to the nearest 0.45 kg) into a feed delivery wagon.

Growth Performance and Dietary Energy Calculations
Growth performance (BW, ADG, gain to feed ratio (G:F), DMI) was determined from receiving through finishing. Cumulative growth performance was based upon initial BW (average BW from initial processing and day 1 with a 4% shrink applied to account for digestive tract fill) and final BW (final BW measurement at day 258 and shrunk 4%). ADG was calculated as the difference between the final shrunk BW and the initial shrunk BW, divided by days on feed. The feed efficiency was calculated from ADG/DMI. Applied energetics measures (observed dietary NE, the ratio of observed-to-expected dietary NE, DMI, and ADG and maintenance coefficient (MQ)) were assessed from days 1 to 258 for the cumulative post-weaning feeding period.
Observed dietary NE was calculated from daily energy gain (EG; Mcal/d): EG = ADG 1.097 × 0.0557W 0.75 , where W is the mean equivalent shrunk BW calculated as: [average of initial shrunk BW and ending period shrunk BW × (478/final BW at 28% empty body fatness (AFBW)), kg (NASEM, 2016)], using shrunk (4%) growth performance. Maintenance energy required (EM; Mcal/d) was calculated by the following equation: EM = 0.077BW 0.75 (Lofgreen and Garrett, 1968) where BW is the mean shrunk BW (average of initial shrunk BW and ending period BW). Using the estimates required for maintenance and gain, the observed dietary NE M and NE G values (Owens and Hicks, 2019) of the diet were generated using the quadratic formula: 877DMI, and NE G was determined from: 0.877NE M -0.41 (Zinn and Shen, 1998;Zinn et al., 2008). The ratio of observed-to-expected NE ratio was determined from observed dietary NE for maintenance or gain/tabular NE for maintenance or gain. Expected DMI was determined based on observed ADG and equivalent BW according to the following equation (NASEM, 2016)

Carcass Trait Determination
Steers were harvested after 258 d on feed post-weaning; steers were shipped the afternoon following final BW determination and harvested the next day at a commercial harvest abattoir. Steers were commingled at the time of shipping and remained this way until harvest time. Liver abscess prevalence and severity were determined following evisceration according to the Elanco Scoring System (Elanco Animal Health; Greenfield, IN) as Normal (no abscesses), A− (1 or 2 small abscesses or abscess scars), A (2-4 well-organized abscesses less than 1 in diameter), or A+ (1 or more large active abscesses greater than 1 in diameter with inflammation of surrounding tissue). Hot carcass weight (HCW) was captured immediately following the harvest procedure. Video image data were obtained from the packing plant for rib eye area; 12th rib fat thickness; kidney, pelvic, and heat fat (KPH); and USDA marbling scores. Yield grade (YG) was calculated according to the USDA regression equation (USDA, 1997). Dressing percentage was calculated as HCW/(final BW × 0.96). Estimated empty body fat (EBF) percentage and final BW at 28% EBF (AFBW) were calculated from observed carcass traits (Guiroy et al., 2002), and proportion of closely trimmed boneless retail cuts from carcass round, loin, rib, and chuck (Retail Yield, RY (Murphey et al., 1960)).

Management of Pulls and Removals
All steers that were pulled from their home pen for health evaluation were then monitored in individual hospital pens prior to being returned to their home pens. When a steer was moved to a hospital pen the appropriate amount of feed from the home pen was removed and transferred to the hospital pen. If the steer in the hospital returned to their home pen, this feed remained credited to the home pen. If the steer did not return to their home pen, all feed that was delivered to the hospital pen was deducted from the feed intake record for that particular pen back to the date the steer was hospitalized. Two steers were removed from the Control group, two steers were removed from the DFM group, three steers were removed from the YCW group, and five steers were removed from the DFM + YCW group. No DFM + YCW interaction was noted for any health outcomes (P ≥ 0.14) in the present study.

Statistical Analysis
Growth performance, carcass traits, and efficiency of dietary NE utilization were analyzed as a randomized complete block design using the GLIMMIX procedure of SAS 9.4 (SAS Inst. Inc., Cary, NC) with a pen as the experimental unit. The model included the fixed effect of DFM, YCW, and their interaction; the block (location) was included as a random variable. Least squares means were generated using the LSMEANS statement of SAS and treatment effects were analyzed using the pairwise comparisons PDIFF and LINES option of SAS 9.4. Distribution of USDA Yield and Quality grade data as well as liver abscess prevalence and severity were analyzed as binomial proportions in the GLIMMIX procedure of SAS 9.4 with fixed and random effects in the model as described previously. An α of 0.05 or less determined significance and tendencies are discussed between 0.05 and 0.10.

Climatic Variables
Ambient weather conditions during the course of the study are presented in Table 2. The average THI during the course of the 258-d study was 43.50. In the present study, only 216.2 mm of precipitation occurred from October 21, 2020, to July 6, 2021. Total precipitation during the course of this experiment was appreciably less than historical records. For 98% of the experiment, the THI was lower than 72. However, two separate heat events occurred during the course of the experiment. Heat event 1 occurred between days 225 and 252 (mean period THI = 69.87) of the present study in which the average THI was greater than 72 for 8-d of the 28-d period.
Heat event 2 occurred between 253 and 258 (mean period THI = 72.70) in which the average THI was greater than 72 for 4-d of the 6-d period. As there were only 12 d in which the average THI was greater than 72, it appears cattle were not subjected to a high-ambient heat load during the course of the experiment.

Respiration Rate and Panting Score
A DFM + YCW interaction was noted for the proportion of steers categorized as PS 2.0 at 1100 h on day 21 (P = 0.03; Figure 1) and respiration rate on day 21 at 1400 h (P = 0.02; Figure 2). No other interactions between DFM and YCW were noted for any other heat stress measurement (P ≥ 0.08). Steers from Control had more steers classified at PS 2.0 at 1100 h on day 21 compared to steers from DFM or YCW (P ≤ 0.05), while steers from DFM + YCW were intermediate, not differing from others (P ≥ 0.10). On day 21 steers from DFM + YCW had greater (P < 0.05) respiration rate compared to steers from DFM, steers from Control and YCW were intermediate, not differing from others (P ≥ 0.10).
Steers from DFM tended to have an increased respiration rate by 7.5% (P = 0.08) compared to steers not fed DFM at 0700 h on day 21. However, steers from DFM tended (P = 0.10) to have a reduced respiration rate by 8.7% compared steers not supplemented with DFM at 1700 h on day 21. Supplementation of DFM had no other appreciable influence on any measures of heat stress in the present study (P ≥ 0.12). Steers from YCW had a 78.3% reduction in the proportion of steers classified as PS 2.0 at 1100 h on day 1 (P = 0.04). Steers from YCW had a 9.4% reduction in respiration rate compared to steers not supplemented with YCW at 1100 h on day 21 (P = 0.01). Supplementation with YCW tended (P ≤ 0.10) to increase PS 1.0 at 1700 h by 8.8% and reduce PS 2.0 by 61.5% at 1700 h on day 21. Steers from YCW tended (P = 0.08) to have a reduction in the proportion of steers classified as PS 1.0 by 13.6% at 1400 h on day 22. The use of YCW had no other influence on any heat stress measures in the present experiment (P ≥ 0.12).

Growth Performance and Dietary Net Energy Measures
Cumulative growth performance responses are presented in Table 3. No DFM + YCW interaction was noted (P ≥ 0.27) for live weight, ADG, DMI, G:F, observed dietary NE, the ratio of observed-to-expected dietary NE, DMI, and ADG or MQ. The use of DFM had no influence (P ≥ 0.15) on live weight, ADG, DMI, G:F, observed dietary NE, the ratio of observedto-expected dietary NE, DMI, and ADG or MQ.
Steers from YCW had reduced (P ≤ 0.04) intake by 1.8%. Observed dietary NEm and NEg tended (P ≤ 0.07) to be increased by 1.2% to 1.5% for YCW compared to nonsupplemented controls. The ratio of observed-to-expected dietary NEm and NEg tended (P ≤ 0.07) to be increased by 1.0% to 1.5% for YCW compared to non-supplemented steers. The ratio of observed-to-expected DMI tended (P = 0.08) to be decreased by 1.0% for YCW compared to non-supplemented steers. The ratio of observed-to-expected ADG tended (P = 0.08) to be decreased by 2.0% for YCW compared to nonsupplemented steers. The MQ was reduced by 3.9% when YCW was supplemented compared to non-supplemented controls.

Carcass Traits
Carcass trait responses are presented in Table 4. A DFM + YCW interaction (P = 0.02) was noted for the distribution of USDA yield grade (YG) 1 carcass. Steers from the control had a greater (P ≤ 0.05) proportion of carcasses classified as YG1 compared to all other treatments. Additionally, a DFM + YCW interaction (P = 0.04) was noted for the distribution of USDA Prime carcasses. Steers from DFM + YCW had a greater (P ≤ 0.05) proportion of carcass classified as

Climatic Variables, Respiration Rate, and Panting Score
The THI can be used as a measurement to analyze the relationship between climate variables and heat stress measures (Lockard et al., 2020). It has been shown to be a good indicator of heat stress severity (Lockard et al., 2020). THI levels greater than 74 have been associated with causing heat stress in cattle (Salinas-Chavira et al., 2015), and values between 70 and 74 for an extended period of time indicate that cattle could experience heat stress (Lockard et al., 2020). The present study evaluated levels greater than 72, due to a wide range of temperatures experienced throughout the duration of the study (−32.78 °C to 36.08 °C). There were only 12 d where the average THI was greater than 72 for the duration of the study, which coincided with the RH supplementation period. This is consistent with the other data in the Northern Plains where limited days were observed with high THI values (Smith et al., 2021). Limited heat stress effects were observed in the present study. In a 76 d study in southeast Texas, steers were exposed to THI levels above 70 for 29 consecutive days (Lockard et al., 2020). During this time, decreased energy intake, ADG, Figure 1. The proportion of steers with PS 2.0 at 1100 h day 21 of RH supplementation for steers fed no direct-fed microbial (DFM) or yeast cell wall (YCW) product (Control); fed the DFM and fed no YCW (DFM; Certillus CP B1801 Dry; Bacillus subtilis, Lactobacillus plantarum; 28 g/steer·d −1 ); fed no DFM and fed YCW (YCW; Celmanax; 18 g/steer·d −1 ); fed the DFM and the YCW (DFM+YCW; Certillus 28 g/steer·d −1 and Celmanax 18 g/steer·d −1 of Celmanax). a, b means differ (P ≤ 0.05).

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and feed efficiency were observed. Steers that were offered a yeast-based additive complex had improved feed efficiencies, which seems to follow the same trend in the present study. Sanchez-Mendoza et al. (2015) also discussed under certain condition yeast supplementation may help to reduce fluctuations in daily feed intake. Perhaps yeast-based additives may help mitigate the effects of heat stress by not reducing performance. However, the THI values observed during the experiment proved that cattle were finished under favorable climatic conditions. This is further confirmed by the measures of respiration rate and PS, which overall classified their values as being non-heat stressed conditions.

Growth Performance and Dietary Net Energy Measures
This study did not note any appreciable differences in growth performance measures. Similar to these findings, Salinas-Chavira et al. (2015) also observed no effects on growth performance during a 139 d period in the desert Southwest, while Lockard et al. (2020) observed no effects on growth performance during a 76 d period in southeast Texas. Pancini et al. (2020) also noted little to no differences when Angus steers were supplemented with a yeast culture product with monensin in Virginia for 150 d or 160 d. Although no differences in the present study were observed during the receiving phase (6.2 kg for all treatments; P ≥ 0.60) when crossbred steers in New Mexico were offered an enzymatically hydrolyzed yeast product and Bacillus subtilis product, increased DMI was observed during the first 3 wk of the receiving period (Colombo et al., 2021).
In contrast to the present study, reports of increased DMI and ADG have been observed in crossbred steers in the desert Southwest that were offered an enzymatically hydrolyzed yeast product (Salinas-Chavira et al., 2015) plus a chromium-enriched yeast product (Sanchez-Mendoza et al., 2015), as well as in calffed Holstein steers in the desert Southwest (Salinas-Chavira et al., 2018). The increased ADG was attributed to increased DMI. Although decreased DMI was observed for YCW steers in the present study, there were no differences in ADG or feed efficiency. Perhaps this provides a possible explanation for the tendency of improved observed-to-expected values for steers offered the YCW supplement. It seems improvements in ADG can be attributed to differences in increased energy intake, rather than improved energy efficiency (Sanchez-Mendoza et al., 2015). It is important to note that Salinas-Chavira et al. (2015) observed increased DMI when the average THI was 80, and Salinas-Chavira et al. (2018) observed increased ADG when the average THI was 75.2. Since cattle in the present study were not under high ambient heat stress during this time, this may be an indicator that cattle supplemented with yeast products may respond better under conditions of stress. This could be in part due to yeast supplementation making ruminal fermentation more stable through the reduction in the variation of ruminal ammonia concentrations and increased concentrations of cellulolytic bacteria (Harrison et al., 1988). This reduction in the fluctuation of ruminal fermentation could be an agent in promoting overall ruminal health.

Carcass traits
No appreciable differences were observed in the present study, which is consistent with studies evaluating enzymatically hydrolyzed yeast cell effects on carcass traits (Salinas-Chavira et al., 2015; Lockard et al., 2020Pancini et al., 2020). However, one study indicated that increased ADG translated to increased HCW in calf-fed Holsteins offered the enzymatically hydrolyzed yeast cell product (Salinas-Chavira et al., 2018). Another study also noted increased dressing percentage and tendencies for lower calculated YGs and a lower percentage of A+ liver scores in control blackhided steers compared to steers offered a yeast-based additive complex (Lockard et al., 2020). When crossbred steers were supplemented with a yeast product with chromium, steers had increased REA, and tendencies for decreased fat thickness, and increased retail yield (Sanchez-Mendoza et al., 2015). However, it was noted the differences were likely due to the addition of chromium with the yeast product component.

CONCLUSION
The use of DFM and YCW had minimal effects on overall growth performance or carcass traits. Cumulative growth performance measures may be enhanced by the use of YCW. The basis for this improvement requires further study but may be attributed to cattle responding to yeast supplementation under conditions of stress, which could have influenced ruminal health through the reduction of ruminal bacterial fluctuations and may have resulted in a tendency for a reduced MQ requirement. However, the DFM and YCW used alone or in combination in relatively unstressed animals cannot be expected to show productive performance benefits. Further investigation is warranted to determine the overall effects on performance and heat stress measures of cattle offered enzymatically hydrolyzed YCW components.

DISCOLSURES
Authors E.R.G., W.C.R., and .Z.K.S. declare no conflict of interest other than the fact that Arm & Hammer Animal Nutrition provided partial funding for this research. Authors Retail yield as a percentage of HCW according to Murphey et al. (1960).

DATA AVAILABILITY
Data can be made available with a reasonable request to Z.K.S.